Wireless communication systems typically include a mobile switching center (“MSC”) and a plurality of base stations (“BS”) coupled to the MSC. Each of the base stations provides wireless communication services to mobile stations (“MS”) within geographical coverage areas referred to herein as cells. Associated with each wireless communication system are communications channels over which user traffic may be transmitted between the base stations and the mobile stations—those channels being derived from a portion of the radio frequency spectrum allocated to wireless communication applications.
To more effectively utilize the available frequency spectrum, wireless communication systems generally have incorporated multiple access techniques, such as frequency division multiple access (“FDMA”), time division multiple access (“TDMA”) and code division multiple access (“CDMA”). In FDMA and TDMA based systems, the frequency spectrum is partitioned into sets of narrow frequency bands (e.g., 30 kHz). In FDMA based systems, each narrow frequency band is used to define a traffic channel over which user traffic may be transmitted between a base station and a mobile station. In TDMA based systems, a traffic channel is defined by a narrow frequency band with time slots assigned for individual call connections. Thus, in TDMA based systems (as opposed to FDMA), more than one traffic channel may be defined using the same narrow frequency band. Typically, in FDMA and TDMA based wireless communication systems, base stations use distinct and unique narrow frequency bands to reduce co-channel interference.
In CDMA based wireless communication systems, the frequency spectrum for a given base station includes only one wide frequency band, typically having a bandwidth of 1.25 MHz or wider. A traffic channel is defined, in part, by the wide frequency band and unique codes associated with individual users or subscribers. Although traffic channels in neighboring cells may use the same wide frequency band, co-channel interference (between base stations in neighboring cells) is reduced as a result of spreading gain attributable to the unique direct sequence codes which define the traffic channels. If the available frequency spectrum is partitioned into two or more wide frequency bands, co-channel interference may be further reduced by using different wide frequency bands to define traffic channels in neighboring cells.
Regardless of the multiple access technique employed by the wireless communication system, a traffic channel needs to be assigned to a mobile station before any call is placed by or to the mobile station. Specifically, the traffic channel is assigned to the mobile station by the base station associated with the cell in which the mobile station is physically located. As long as the mobile station stays within the same cell, the mobile station may use the same traffic channel for the duration of the call. If the mobile station moves to another cell during the call, a handoff will be performed between the base station associated with the new cell and the base station associated with the old cell. Handoffs in which a mobile station switches between traffic channels using the same frequency band are referred to herein as intra-frequency handoffs. By contrast, handoffs in which a mobile station switches between traffic channels utilizing different frequency bands are referred to herein as inter-frequency handoffs. It should be noted that inter-frequency handoffs occur in FDMA and TDMA, as well as CDMA based wireless communication systems.
Inter-frequency handoffs in a CDMA system can occur in a variety of circumstances. For instance, a region of high demand, such as an urban area, may require a greater number of frequencies to service demand rather than a lower demand region surrounding it. Sometimes it is not cost effective to deploy all available frequencies throughout the system. Thus, a call originating on a frequency deployed only in the high demand area will incur an inter-frequency handoff as the mobile station user travels to a less congested area.
In another circumstance, an inter-frequency handoff may occur between adjacent service areas, which may involve movement by the mobile between adjacent cells in the same wireless system or between separate, but contiguously located wireless systems. Such adjacent service areas may offer service on different frequencies, which will require the mobile station to be handed off to a different frequency in order to provide call continuity.
One concern associated with inter-frequency handoffs is a tendency for the mobile station to oscillate between two frequencies. This occurs when a mobile station is initially handed off to a new frequency and then subsequently returns to the old frequency. Such a situation is referred to herein as “ping-ponging.” In geographical areas where the old and new frequencies are both characterized by strong RF signals, ping-ponging is a potential problem. Another concern associated with inter-frequency handoffs is the occurrence of dropped calls. Specifically, inter-frequency handoffs are somewhat more likely to experience a failure of the mobile station to establish communications with the base station operating at the new frequency (for a variety of reasons), thereby resulting in dropped calls.
Handoffs between base stations are initiated by a trigger mechanism, and an inter-frequency handoff is predicated on a particular trigger. When the conditions for an inter-frequency trigger are met, a conventional inter-frequency handoff process is initiated. Existing inter-frequency handoff triggers rely on       E    C        I    O  measurements at a particular mobile station—      E    C        I    O  being a ratio of pilot energy per chip to the total received power spectral density at the mobile station antenna.       E    C        I    O  can also be viewed essentially as a digital signal-to-noise ratio. However,       E    C        I    O  measurements at fixed geographic locations are subject to extreme variations due to traffic loading. For example, for a one-cell mobile communication system, during unloaded or lightly loaded conditions, the mobile received             E      C              I      O        ,may be −3 dB, while during high loading, the mobile received       E    C        I    O  at the same location may be −10 dB. In order to avoid situations where no inter-frequency handoff trigger is reached during low traffic loading, the       E    C        I    O  based trigger must be set to a very high ratio, typically on the order of −3 dB. However, this can result in an unnecessary amount of inter-frequency handoffs occurring during high traffic loading and a significant reduction in the offered traffic load. Lower ratio triggers will increase dropped calls due to the unpredictability of loading, but may result in higher offered traffic loads.
Therefore, there exists a need for improving the reliability of inter-frequency handoffs, to consequently minimize the number of dropped calls, to minimize the occurrence of ping-ponging and to increase the offered traffic load by developing a more reliable and accurate inter-frequency handoff trigger.